Malignant transformation by the avian Rous sarcoma virus is induced by a virus-coded protein (pp60src) which plays no role in virus reproduction. Attempts have been made to identify the metabolic function of this protein in transformation. Characteristic morphological and metabolic features of transformed cells have been used as a basis for examining possible functions of the transforming protein and a virus mutant which induces temperature dependent transformation has been useful in distinguishing between direct and indirect effects of the protein. Using these methods we have been able to separate late changes in phenotypic transformation from earlier changes induced by the functioning virus-coded protein and have eliminated from consideration several metabolic areas formerly implicated as primary virus-induced changes. However, an increased synthesis of ATP and utilization of energy by transformed cells has been observed, which may be a direct result of the virus-coded protein. An increased synthesis of macromolecules apparently is not responsible for the increased energy utilization, and the basis for this change is being pursued.